The invention described herein relates generally to methods and apparatus for laser pulse compression, and more particularly to methods and apparatus for producing UV, VUV, or X-ray laser pulses of very short duration, and especially to methods and apparatus for producing such laser pulses having high power.
There is a need for the production of very short laser pulses in many areas of science and technology. This need extends to laser pulses of short wavelength. For example, temporally short UV or VUV laser pulses can be beneficially used in the fields of plasma physics and multiphoton chemistry, and the pulsed X-ray laser holography of living cells has been proposed. In this last application it is believed that the heating caused by the incident X-ray laser pulse will cause a thermal molecular motion that will smear the details of the measurement, if the X-ray laser pulse is of a duration longer than about 20 picoseconds. Unfortunately, the conventional methods for shortening laser pulses, such as those employing pockels cells or opto-acoustic modulators, rapidly lose their utility at wavelengths shorter than ultraviolet.
A means for modulating or controlling the positioning of laser output is disclosed by Mego, Jr. et al, in U.S. Pat. No. 3,975,692 issued Aug. 17, 1976. The invention comprises the introduction of a fluidic jet or a flowing liquid stream, preferably of dichlorodifluoromethane, either into the optical path within a laser cavity, or into the optical path of a continuous wave laser beam. Because it is limited by the relatively slow velocities of fluidic jets and flowing liquid streams that slowly change with time, this method is not adaptable to the production of very short laser pulses, appreciably less than one nanosecond in duration. Also, the natural opacity of the materials comprising the jets and streams, makes the method unsuited for use at laser wavelengths in the ultraviolet to X-ray range.
A means of infrared pulse compression is disclosed by Corkum in U.S. Pat. No. 4,612,641 issued Sept. 16, 1986. In this process, an infrared optical pulse is chirped and then passed through a dispersing means, which causes pulse compression. This infrared technique does not have applicability at UV, and shorter, wavelengths.
An optical switching element for long wavelength, 10 micron radiation is taught by Bjorklund in U.S. Pat. No. 4,166,254 issued Aug. 28, 1979. The element is a diffraction grating formed from an array of color centers in an alkali halide crystal. The grating is switched, by an external pump laser, so that it transmits radiation of a certain wavelength when the color centers are in the ground state, and diffracts that radiation when the color centers are in an excited state.
Genack in U.S. Pat. No. 4,486,885, issued Dec. 4, 1984, describes a system for use with a long wavelength continuous wave CO.sub.2 laser beam in which the phase of the coherent light is rapidly changed, as it impinges on an absorbing cell containing carbon dioxide, to produce a short amplified pulse of light.
The production of very short laser pulses in a synchronously pumped, continuous wave dye laser is treated by Mourou et al in U.S. Pat. No. 4,517,675 issued May 14, 1985. The laser medium is a mixture of a laser dye and a fast saturable absorber. The disclosed method is not applicable at ultraviolet and X-ray laser wavelengths.
Apparatus, for controlling the pulse shape of a laser pulse, comprising a plurality of Kerr-effect gates that are selectively triggered by a control laser pulse, is disclosed by Siebert in U.S. Pat. No. 4,061,985 issued Dec. 6, 1977. The nature of the physics involved in this scheme is not appropriate to ultraviolet to X-ray laser wavelengths.
It is presently recognized that the first operational laboratory X-ray laser is taught by Rosen et al in Physical Review Letters 54, 106 (1985), and experimentally demonstrated by Matthews et al in Physical Review Letters 54, 110 (1985). This seminal X-ray laser system is also fully disclosed by Campbell et al in U.S. patent application Ser. No. 676,338 filed Nov. 29, 1984. A related short wavelength laser, whose output extends into the EUV and X-ray region, is taught by Hagelstein in U.S. Pat. No. 4,589,113 issued May 13, 1986. The short wavelength laser pulses produced by these laser systems have durations on the order of a nanosecond. The prior art techniques of pulse compression, discussed in the preceeding paragraphs, have applicability to long wavelength laser pulses, but cannot be used to compress or shorten the duration of nanosecond time scale, very short wavelength laser pulses.
Thus, the problem remains of producing ultraviolet or X-ray laser pulses of duration in the picosecond range. It would especially be a significant advantage to produce high power ultraviolet or X-ray laser pulses having a duration in the picosecond range.